Containers produced by this method present a series of advantages as as follows thin walls, high uniformity, high ductility with no residual stresses, non-reactive with the pressed powders, high weldability, easy control of final dimensions of the product pressed, and wide applicability for complicated shapes. The as-pressed product with these containers presents high quality with no distortion or superficial defects.
The Hot Isostatic Pressing (HIP) of metallic, ceramic or combination of metallic and ceramic powders, is a standard technology in the hot consolidation of the powder metallurgy.
In this technology, the powder to be pressed is put in a container which has the shape of the mass of powders after compacting. Said container is normally degassed and hermetically sealed after filled with powders. Pressure and heat are applied to the container in all directions inside autoclaves, in such way the container is collapsed against the powder to highly densify said powders. In other words, the containers at high temperatures act as a pressure transmission media, and the powders receive the force applied to the container. Simultaneously, the applied heat produces the sintering of compacted powders.
After the HIP process, the container is removed from the compacted mass of powders which is further processed by one or more steps of finishing such as forging, machining and/or heat treatment in order to produce the desired final shape. One of the most critical elements in HIP is just the nature and characteristics of the containers used. The container, must withstand and transmit high pressures toward the mass of powders. The container must be flexible or deformable while keeping its structural integrity at high temperatures and high pressures without cracking. The container must be non-reactive with the powders being pressed and capable of hermetic sealing and in some cases vacuum evacuation without distortion. The type of container used in a HIP process, determines the dimensional precision that the final pressed part can reach.
Due to the high costs of forging and machining operations, there has been special attention given toward the development of containers for HIP process that would be able to produce compacted mass of powders with high dimensional precision and with minimum final finishing operations required.
Containers for HIP processes are made normally of glass, ceramics and metals. The type of metal is selected according to the characteristics of powders to be pressed, considering temperature and pressure needed for the HIP process and reactivity between container and powders.
For the simplest shapes as billets, bars, cylinders, etc., the surface defects produced by the welding zones are eliminated by machining increasing the cost of the pieces being processed. In complex shapes is almost impossible to eliminate such welding zones that produce surface defects. A great drawback of metallic thin wall containers, when they are produced by stamping, extrusion, deep drawing and/or welding, is that they produce distortions on the compacted mass of powders. That is due to a non-uniform shape reduction of the container caused by the different stress accumulation produced during the container manufacturing. For that reason, the final shape so obtained will be totally different as compared with the initial container shape.